Make "dumpsys tethering" print the number of registered callbacks.

Test: manual
Bug: 177265744
Change-Id: I07f56459125d5629dcafe5683eecb42d0ef9e2bd
diff --git a/Tethering/src/com/android/networkstack/tethering/Tethering.java b/Tethering/src/com/android/networkstack/tethering/Tethering.java
index b52ec86..7cb31bd 100644
--- a/Tethering/src/com/android/networkstack/tethering/Tethering.java
+++ b/Tethering/src/com/android/networkstack/tethering/Tethering.java
@@ -2336,6 +2336,9 @@
         pw.println("Tethering:");
         pw.increaseIndent();
 
+        pw.println("Callbacks registered: "
+                + mTetheringEventCallbacks.getRegisteredCallbackCount());
+
         pw.println("Configuration:");
         pw.increaseIndent();
         final TetheringConfiguration cfg = mConfig;